CHARACTERISTICS, DOSIMETRY & MEASUREMENT OF EMF

Transcription

1 WHO Meeting on EMF Biological Effects & Standards Harmonization in Asia and Oceania October, 2001, Seoul, KOREA CHARACTERISTICS, DOSIMETRY & MEASUREMENT OF EMF Masao Taki Tokyo Metropolitan University Soichi Watanabe and Kanako Wake CRL JAPAN

2 Roles of Engineering in EMF Health Issue Characterize external EMF precisely Measure the field correctly Provide insight into the interaction of EMF with cells, organs, and body (Dosimetry) Design and install the exposure setups for experiments on biological effect, and control their quality Develop guidelines based on biological data Assess the compliance with guidelines

3 Characterization

4 Characterization of EMF EMF Electric field (E,D) : vector function E(t,r) Magnetic field (H,B) : vector function H(t,r) Waveform Sinusoidal waves (frequency, amplitude, phase) Non-sinusoidal waves (spectrum=amplitude and phase at any frequencies) Polarization Orientation of E and/or H field vectors

5 Sinusoidal and Non-sinusoidal Waves Amplitude spectrum Waveforms with 5-harmonic

6 Spectrum of EMF Frequency [Hz] Wavelength [m] Non-ionizing Radiation (wavelength < 100nm) LF khz RF 100kHz 300GHz ELF - 300Hz IF 300Hz-10MHz RF 10MHz 300GHz

7 Radiation of EMF E-field H-field

8 Polarization Orientation of the field vector In far field region E and H are perpendicular to each other. Orientation of E-field Important factor in the coupling between field and body

9 Linear and Circular Polarization

10 Measurement

11 Measurement Small Antennas (broad-band) Infinitesimal electric dipole = Electrical gap Element of E-field sensor Infinitesimal magnetic dipole = Small loop Element of B-field sensor Resonant antennas (narrow-band) Other sensors Pockels effect E-field sensor Hall effect B-field sensor

12 E-Field Measurement Broadband technique Isotropic E-field probe with three orthogonal small dipoles Provides rms values with a certain time constant Well developed technique Easy operations, widely used Some types give weighted sum of rms values following frequency dependence of reference levels of specific guidelines Diode detector type is not suitable for pulsed field measurement (thermocouple type is recommended) up to >10 GHz

13 H-field Measurement Broadband Technique Isotropic probe with three orthogonal small loops Common readout unit with E-field instrument Similar features to E-field probe up to about <1GHz If plane wave condition is applied, H can be obtained by H=E/120π

14 Dosimetry

15 Dosimetry Metrology of dose Dose the amount of a substance we are exposed to or come in contact with Dose in EMF Induced current density Stimulation effect (< 100 khz) Specific Absorption Rate (SAR) [W/kg] Thermal effect Other metric Possible non-thermal, non-stimulation effect (?)

16 Basic Coupling of E and B with body E B

17 Models for Dosimetry Early works Analytical approaches Simple shapes

18 z MRI-Based Models y vertical section x horizontal section at the height of 50 mm x Rat Model Human Model Development of numerical methods - Finite Difference method (LF) - Finite-Difference Time-Domain (FDTD) Method

19 Induced Current Distributions in Human Bx (body axis) By ( left to right) Bz (front to back)

20 Induced Current Distributions in Rat Bx (body axis) By ( left to right) Bz ( front to back)

21 Current Densities in Rat and in Human Whole body Brain Bone Muscle Fat Heart B x (body axis) B y (left to right) B z (front to back) Max. Ave. Max. Ave. Max. Ave. Rat Human Rat Human Rat Human Rat Human Rat Human Rat Human Currents induced in human are larger than currents in rat because of the size difference B: 50 Hz, 1 µt. J [µa/m 2 ]

22 Recently Developed Human Models Japanese male and female Spatial resolution:2 mm Identified tissues:> 50

23 Resonance of Whole Body SAR H E k E-Polarizatoin Average SAR, W/kg Grounded Ungrounded frequency, MHz

24 SAR distributions and Effect of Grounding Contact d = 1 cm d = 10 cm d = 50 cm Free space SAR distribution (Computed at 37 MHz)

25 Limb Current in MHz Reference levels for current induced in any limb in MHz Current (ma) Occupational exposure 100 General public 45

26 Equivalent Antenna for Limb Current Measurement

27 SAR Measurement SAR = ρ σ E 2

28 Realistic Head Phantom (Standard Japanese) Lossless spacer With ear Ear affects SAR distribution but does not increase maximum local SAR

29 Development of Exposure Setups

30 Assessment of Experimental Setups Quantify exposure condition Important for the improvement of reproducibility Both numerical and experimental exposure assessment 1 Air Ventilation Duct SDRatinaTube Stopper SAR [W/kg] rats 4 rats 2 rats Results of numerical calculations

31 Exposure setup and rat phantoms. Experimental SAR Measurement Thermograph Method Thermography camera Divided phantom

32 Exposure Setup for Long-term Cancer Promotion Study ENU-initiated brain tumor of rats Two-year NTP study at a GLP facility Localized exposure in the brain with least whole-body SAR Many animals

33 Numerical Dosimetry X-ray CT images Numerical models Calculated SARs 126 g 263 g 359 g Growth

34 Experimental Dosimetry Phantom for Termograph Method phantom recipe [g] Water Ager 52.3 NaCl 7.52 NaN TX Polyethylene

35 Comparison between Numerical and Experimental Dosimetry Calculated SAR SAR by Thermography SAR[W/kg] Numerical Experimental Depth[mm]

36 Exposure Setup for Brain Circulation Study SD rat with Cranial Window Allows in situ observation of circulation Very localized exposure in the brain by a loop antenna Small loop

37 Electromagnetic Field Distribution Measured E-Field Computed Measured H-Field Computed

38 SAR Distributions FDTD Calculation Measured by Thermography

39 Exposure Setup for MW Effect on Eye 2.45 GHz Dielectric-loaded waveguide antenna Pulsed field/cw Stub tuner Generator Antenna

40 In Vitro Exposure Setup

41 What else engineering could contribute? Exercises for engineers and physicists

42 (1) Microwave Hearing (MWH) Buzzing or clicking auditory sensation caused by the exposure to high peak power microwave pulses on the head Thermo-elastic waves generated by the small but rapid local heating should cause the sensation. An established phenomenon specific to pulse modulated microwaves.

43 Questions What is the mechanism? Thermoelastic waves (J.C. Lin, 1970 ) Does the heterogeneity of head affect the phenomenon? What waveform do we actually perceive? How strong is the acoustic waves in the brain?

44 SAR Distribution and Elastic Waves Local SAR [W/kg] Amplitude, power, displacement of MWH is very small Acoustic resonance in the head governs the waveform

45 (2) Intrinsic Current Cells lying near an axon on which action potentials propagate are exposed to strong field Are these cells affected by the field?

46 (3) Circularly Polarized Magnetic Field Kato et al. (1994) reported inhibition of melatonin secretion only by exposure to circularly polarized B-field. He did not find any effect for linearly polarized field. Can difference in induced current characteristics explain the difference?

47 Induced Current by Circularly Polarized B

48 How circular the current in brain is? The result is still puzzling us. Needs an experiment for X-y plane rotating field.

49 (4) Demodulation of AM Signals Demodulation of the Baseband Component due to Nonlinearity Input Local SAR of 10W/kg Basic restriction on local SAR for Occupational exposure Internal E-field ~100V/m (σ RF ~1 S/m) Nonlinear response Output Threshold of Magnetophosphene ~10mA/m 2 (σ ELF ~1 S/m) ~0.01V/m?

50 SAR and E-Field Distribution by RF 5cm 900 MHz Std. Dipole SAR E-field Antenna input 1 W Peak Local SAR (10 g av.) 1.35 W/kg Peak SAR near retina 1.3 W/kg

51 Result of Search for MW Phosphene So far no phosphene-like phenomenon has been found. Induced current by modulated RF may differ from that by ELF magnetic field. However, we know that the threshold current density for electrophosphenes is similar to magnetophosphenes.?????? Induced by E or RF Induced by B

52 (5) Exposure Assessment of Epidemiological Studies >4 mg <4mG Epidemiological Studies RR~2.0 What dose-effect relationship could explain the elevation of RR? How strong should the effect be to explain the RR??

53 Laboratory Studies Volunteer Study? Animal Studies ~1???

54 Concluding Remarks We would like to emphasize the importance of engineering and physics in EMF health research. Good engineering provides: Quantitative understanding of the events in the body Hints of mechanisms of interaction Reproducible experiments Means to hypothesis driven experiments We can ask a number of good questions to ourselves which could be answered by investigations based on engineering, physics, and mathematics.